In order to eliminate the noxious substances contained in the air supplied to a compartment occupied by people, it is first necessary to detect them and then to produce signals or control outputs in order to switch on or off--the filtering systems and/or the ventilation systems.
It has already been proposed to provide analyzers, which however are comparatively expensive to acquire and to operate, not to mention that they are capable of detecting only separate components of noxious substances.
Further, it is known to use semiconductor sensors for these purposes, which meet the general requirement to respond to the usual air pollutants. However, the range of responsiveness of these sensors lies well above the objectionable level of concentration of many noxious substances to which people are subjected. Besides, semiconductor sensors are sensitive to temperature fluctuations and differences in flow velocity of the air streams to be analyzed. Furthermore, the response time of these sensors, particularly in the case of low concentrations, and their decontamination time are too high for their use for instance for the control of filter systems and/or ventilation systems in motor-vehicle cabs.